Slight twitch in Yaw

I have been flying my P2V for about 18 months and really enjoy it. I now fly it in NAZA mode which I think is much better particularly being able to use the HL and ATTI modes when filming. About 3 months ago I fitted the DroneExpert Gimbal and it works really well. However now that the filming is so stable I notice a slight twitch or jig in yaw occasionally. At first I thought it was pilot induced when I was climbing or descending but it happens in level flight or even the hover sometimes. I have fully recalibrated the controller using the latest Assistant software and the ICU and compass and of course calibrated the compass before flight.

The compass is well below the line of the gimbal servo , so has anyone else experienced this and better still solved it?